Accidentally hit "toggle full screen", How do I exit?
To toggle out of full screen area (what I think you did) press AltF10, or move your mouse to the top right corner and click the button that appears.
Here I start in full screen area, notice the button that appears in the top right corner.


If you just maximized the window you can get back to the normal view a number of ways.


Click the "Back To Previous" button in the info window at the top.
Press CtrlUp Arrow
Press CtrlDown Arrow
Press ShiftSpace


If you toggled blender into full screen view press AltF11.



Update
For blender 2.8 the shortcuts are 

&#9096; Ctrl + Spacebar to toggle maximize

or 

&#9096; Ctrl + &#9095; Alt + Spacebar
 to toggle Full Screen.